Xerographic reproduction apparatus use a photoreceptor in the form of a drum or a belt in the creation of electrostatic images upon which toner is deposited and then transferred to another belt or drum, or to paper or other media. Once the toner image is transferred, most xerographic apparatus clean the photoreceptor in ways that can abrade the surface, changing the thickness of the photoreceptor over time. Even without such abrasion, the thickness of the photoreceptor will decrease through use over time, typically through contact friction with various other devices in the system, such as the transfer roller. See, for example, the “Nominal” curve in FIG. 6, which shows a graph of charge transport layer thickness reduction versus print count. Because of the nature of the photoreceptor, a change in its thickness will result in a change in its electrostatic performance.
After enough of the surface layer of the photoconductor has been worn away, print quality defects will typically begin to appear. For example, with organic photoconductor drums, charge depleted spots (CDS) can appear in the output prints after enough of the photoconductor outer layer, which is the charge transport layer (CTL), has been worn away. To avoid these sorts of defects, some xerographic devices use a page counter and simply stop using the photoconductor, or at least signal that the photoconductor should be replaced, after a predetermined number of prints have been made. Since photoconductors are typically somewhat expensive to replace, the life of these devices can have a significant impact on the overall run cost of the print engine. In fact, this can be one of the largest contributors to the parts costs for many tandem color xerographic machines.
Many xerographic engines, particularly color xerographic engines, make use of contact and/or close proximity AC charging devices, such as biased charging rollers (BCRs), such as seen in FIGS. 1-3. Contact and/or close proximity type charging devices typically use an AC waveform with a DC offset bias to exceed the required threshold voltage for air breakdown, VTH, which varies with the particular geometry of the print engine, thereby generating the desired photoreceptor charging behavior. Although the device itself may contact the photoreceptor, contact is not a necessary condition for the corona to contact or reside in close proximity to the photoreceptor and lead to high rates of photoreceptor wear. Therefore, charging devices with air gaps between the surface of the device and the photoreceptor can also benefit from embodiments disclosed herein.
A typical response of the photoconductor potential as a function of the AC peak-to-peak voltage charging actuator is shown in FIG. 4. The location of the actuator saturation point in this curve is typically referred to as the “knee” of the charge curve (the point at which further increases in the actuator do not significantly affect the output photoconductor charge voltage). Typically, non-uniform print quality is obtained for AC charging devices when the AC peak-to-peak actuator is operated below this knee value. In addition, under certain conditions, some print quality defects may occur for actuator value close to, but still slightly above, the knee of the charge curve. One type of defect that can occur is a light and dark spots pattern (similar to a salt-and-pepper noise) that occurs between the charging knee and a Vp-p value known as the background disappearing point (“BDP”). The speckles that appear as a result of the BDP defect are typically referred to as BDP spots. To prevent BDP spots from occurring, it is necessary to maintain the AC charging actuator at a value safely above the BDP. Thus, in most xerographic engines that make use of contact and/or close proximity AC charging devices, the charging actuator is operated at a value sufficiently far above the knee of the curve to ensure acceptable output print quality despite variations in the process.
While the BDP spots defect appears to cease to occur after a number of prints have been run, on the order of several thousand or more, depending on the particular xerographic engine and/or photoconductor, eliminating the defect from the first print is preferred. The age related effect means that, while it is necessary to steer the AC actuator slightly higher than the BDP value early in the life of the photoconductor, it is possible to reduce the AC charging actuator toward the knee of the charging curve once a particular threshold in print count has been reached.
In xerographic systems using contact and/or dose proximity AC charging devices, the rate of wear of the photoconductor is accelerated as a result of positive ion deposition onto the photoconductor surface by the charging device. These positive ions are believed to interact with the surface of the photoconductor, thereby making it more susceptible to abrasion and wear. The greater the number of positive ions deposited onto the surface of the photoconductor during charging, the more quickly the photoconductor surface material will wear. In addition, the larger the amount by which the charge knee voltage is exceeded, the larger the amounts of both positive and negative ions that will be produced during each cycle of the charging waveform. This is illustrated, for example, in FIG. 5, which shows simulation results indicating the amount of positive charge deposition onto a photoconductor as the charger actuator voltage increases above the knee value. Thus, the magnitude of the AC charging voltage applied to the charging device can significantly affect the amount of positive charge deposition that occurs on the photoconductor surface. For a given DC offset voltage, larger peak-to-peak amplitudes for the applied AC voltage above the charging knee will typically lead to larger amounts of positive charge deposited onto the PC surface for each charging cycle. Once again, the larger the amount of positive charge deposited onto the photoconductor surface by the charging device, the faster the PC surface will wear. Thus, it is highly desirable to minimize the distance of the charging actuator above the knee of the charge curve at all times.
In many xerographic systems that make use of a contact and/or close proximity AC charging device, the AC charging actuator is not actively adjusted. The AC charging actuator is typically the amplitude of the AC voltage waveform for constant voltage mode charging, or the AC current setting for constant current mode charging. However, the DC offset voltage for the AC charging device is, in many engines, adjusted as part of the normal process controls to help maintain consistent output The AC charging actuator value of many xerographic print engines is determined and set as part of the initial design of the engine. The AC charging actuator thus remains fixed and is not actively adjusted during normal operation. Since print quality defects are known to occur for charging actuator values close to or below the knee, larger design values for the AC actuator are typically chosen to ensure that variations in the process behavior will not result in variations in the charging output voltage. However, these larger actuator values result in more positive ions being deposited onto the photoconductor's surface during each charging cycle (each cycle of the AC waveform). Once again, the wear rate of the photoconductor is related to the amount of positive charge deposition onto its surface, where an increase in positive charge deposition results in a decrease in the expected life of the photoconductor. Thus, a tradeoff is made at design time between the print quality latitude of the charging actuator and the amount of excess positive charge deposited onto the photoconductor surface, and therefore the expected wear rate of the device.
In an effort to limit the amount of positive charge deposited onto the surface of the photoconductor while maintaining acceptable output print quality, some prior methods have attempted to design different AC waveform shapes. Another technique modulates the AC waveform in different ways, and other approaches have been used. However, each of these approaches has focused on altering the design of the AC charging waveform at design time, not making any active adjustments to the AC actuator during normal operation of the print engine.
Instead, to address the need for longer life photoconductor devices in systems with contact and/or close proximity AC charging, many prior methods have focused on materials related solutions. These types of approaches can include such things as improved overcoats on the photoconductors to make them more durable. Unfortunately, these types of solutions are somewhat difficult to develop and can, in fact, cause other problems in the system. For example, creating a harder photoconductor surface in a xerographic system with a blade cleaning device shifts the wear to the cleaner blade, which can lead to reduced cleaning blade lives, which might not allow a significant gain in system run cost to be realized through such a materials based solution.
Still other methods have looked at using non-contact charging devices or other subsystem changes to reduce the abrasion of the photoconductor surface. For example, a non-contact charging device, such as a scorotron, applies high voltage to a wire or pin coronode located a distance, such as about 500 μm or more, from the photoreceptor surface. The charge generating corona discharge is localized around the coronode is such devices, not touching, but in relatively close proximity to the photoreceptor.
Some prior methods, such as, for example, that disclosed in U.S. Pat. No. 7,024,125, have suggested mechanisms for adjusting the charging actuator in an active fashion. However, these prior methods are limited in the information that they use to adjust the charging actuator. Such methods are typically limited to measurement of a current as a mechanism for measuring the charge level of the photoconductor. Unfortunately, for some devices, such as biased-transfer rolls, the measurement of a current using a constant voltage mode of operation can be quite noisy. For example, if the impedance of any component changes, this can have a detrimental effect on the current measurement. In addition, prior methods typically do not make use of image quality information in their adjustment of the charging actuators. Rather, these prior systems are limited to measurements only of the underlying process parameters, namely the location of the charging knee, or threshold voltage, through measurement of a downstream current flow. Thus, there is a need for a xerographic system with an active adjustment scheme that will optimize photoconductor life in a robust fashion while ensuring that charging related print quality defects do not occur.
Embodiments significantly improve the life of a photoconductor in a xerographic engine by actively adjusting the AC charger settings for contact and/or close proximity charging devices used in the engine based on measurements of the charging threshold Vknee and also possibly based on measurements of print quality related parameters. Embodiments actively adjust the AC charging actuator (peak-to-peak voltage or AC current) to reduce the amount of positive charge deposited onto the surface of the photoconductor, thereby extending its life, as illustrated by the “Reduced BCR” curve of FIG. 6 while also maintaining an acceptable distance between the actuator setting and the knee of the charging curve and/or the required print quality defect thresholds to minimize the possibility of charging related print quality defects.
The selection of the contact and/or close proximity AC charging actuator operating value (the actuator) is very important from a photoconductor device life point of view since positive charge deposition onto the PC surface drives the PC wear rate in many xerographic systems with contact and/or close proximity AC charging devices. The charging actuator operating value is the peak-to-peak voltage value of the charging waveform for constant voltage charging devices and is the AC current value for constant current charging devices. Another concern regarding the choice of the AC charging actuator setting is the uniformity of the resultant charged voltage, Vhigh, on the photoconductor. Non-uniformities in Vhigh can translate to undesirable non-uniformities in the output of the xerographic apparatus. Too low of an AC charging actuator value tends to result in these types of non-uniformities in the Vhigh output from charging. Thus, choosing appropriate values of the AC charging actuator according to embodiments can prevent print quality defects from occurring in addition to extending photoconductor life.
To achieve embodiments, a measure of the photoreceptor surface potential is useful. Surface potential can be measured using electro-static voltmeters (ESVs) and/or the thickness of the photoconductor can be estimated using measurements from the BCR or BTR, high voltage power supply, such as with the techniques disclosed in U.S. patent Ser. No. 11/644,277, filed concurrently herewith and incorporated by reference above. However, ESVs can be costly to implement in engines that do not already include ESVs, particularly in color xerographic apparatus including multiple photoreceptors and/or marking engines. U.S. Pat. No. 6,611,665 to DiRubio et al., as well as U.S. patent application Ser. No. 11/644,277, incorporated by reference above, discloses a method and apparatus using a biased transfer roll as a dynamic electrostatic voltmeter for system diagnostics and closed loop process controls. While the techniques disclosed in the '665 patent are useful, they can suffer inaccuracies due to unpredictable aging effects of the elastomers used in the BTR, as well as other factors. Such measurements are more accurate than those obtained by prior methods, such as that disclosed in U.S. Pat. No. 7,024,125 discussed above, which employs a constant voltage mode operation. Thus, the method of using a biased charging roller as disclosed in U.S. patent application Ser. No. 11/644,277, incorporated by reference above, is preferred for accuracy when possible. Using the measures of photoreceptor surface potential (VPC), the knee location can be determined, and embodiments can adjust the AG actuator accordingly. The routine of embodiments can be run periodically, such as during cycle-up or cycle-down or every so many prints, to ensure consistent output of the xerographic apparatus in which it is used.
To achieve embodiments, a measure of the occurrence and/or level of charging related print quality defects is also useful These measurements can be obtained using a variety of techniques and sensors. For example, in situ scan bar sensors can be used in the xerographic printing engine to detect structured image print quality defects, such as CDS and BDP defects. These sensors could be used to detect the occurrence, size, and other properties related to such print quality defects.